mwifiex: avoid double list_del in command cancel path

Command cancel path cancels the current command and moves
it to free command queue. While doing that it deletes the
command entry from the pending list. This is not correct
as the entry has been already deleted from the pending
list at 'mwifiex_exec_next_cmd'. Fixing it.

Also making sure the stale command pointer is cleaned and
unaccessible for later use.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>